The present teachings relate to an inspection system utilizing one or more Solid Immersion Lenses (SILs), the inspection system including silicon lenses, and a surrounding housing.
High spatial resolution imaging, in conjunction with high collection efficiency, can be used in the examination and testing of particular sub-micron structures. The size reduction of these sub-micron structures have been following the prediction made by Gordon Moore in 1965, more commonly known as Moore's Law. Within the semiconductor industry, Moore's Law stated that the maximum number of transistors able to fit within a given area will double every two years. Since the optical numerical aperture is based on the index of refraction of the medium between the objective and sample and the collection angle of the objective, the highest numerical aperture (NA) achievable by an air lens is unity.
This technology utilizes a hemispherical or cone shaped solid immersion lens (Hemi or Cone SIL) made of silicon, or any material with an index of refraction beyond that of air. Since the optical numerical aperture is based on the index of refraction of the medium between the objective and sample as well as the collection angle of the objective, the highest numerical aperture (NA) achievable by an air lens is unity. When this SIL lens is used in conjunction with a corresponding backing objective it will yield an image with both an increased magnification and numerical aperture (NA). As stated above, this increase is brought about by the simple increase in index of refraction of the SIL material versus that of air. Some of the key aspects of this technology are the correct alignment of the SIL lens onto the object under observation (or test), a large solid angle of admitted light, and ability to adjust the displacement of the SIL lens along the chief ray of the backing objective to achieve a large focal range while controlling the forces and, thereby, the stresses imparted onto the device under test (DUT). Currently, there are several known techniques for securely keeping the solid immersion lens in place.
Conventional systems exhibit instability with the landing of the SIL lens with respect to the silicon DUT. The application of downward looking, gravity-fed, and multi-mechanical-spring loaded solid immersion housings exhibit specific characteristics. One conventional example allows the SIL to float within its housing, using gravity to settle the lens upon contact with the Device Under Test (DUT). Thus, when the SIL touches down on the DUT, the housing is lifted from the SIL, removing all sources of thermal contact. The SIL lens simply “sits”, or “is placed”, on top of the DUT. However, due to normally occurring surface irregularities due to the preparation of the surface, the SIL will not properly couple to the DUT, greatly degrading SIL imaging. In addition, the lateral position of the SIL with respect to the DUT cannot be repeatably located.
DUT flatness is important, as gaps between the SIL lens and DUT, brought about by SIL lens tilt or sample preparation, produces aberrations and makes high resolution observation difficult.
In another conventional implementation, pressure is applied and regulated to control the SIL lens with respect to the DUT. A stress/pressure sensor is used as a feedback device to control and adjust the amount of pressure on the DUT.
In the above described conventional systems, the excessive force is typically produced by, among other factors, the lack of effective control of the angle between the solid immersion lens (SIL) and the device under test (DUT). The angle of the solid immersion lens with respect to the backing objective and the device under test, in conventional systems, is typically rigid in the tip and tilt directions of motion. The rigidity is typically due to:                a. conventional lens design attempts to constrain five degrees of motion of each lens with respect to the other (rotation about the chief ray is not controlled—axisymmetric assumption) This results in a rigidly fixed SIL lens, which has little angular compliance, and        b. the spring rate of the “SIL holder” along the direction of the chief ray, as well as in the tip and tilt directions, is very difficult to reduce. This is due to the traditional use of coil-over designs, which require some preload force, and tight clearance components, which slide with respect to each other, to avoid backlash. Backlash degrades image performance due to non-repeatable lateral displacements.        
In conventional systems utilizing “coil (spring) over” designs, the preload of the spring must be overcome before the SIL can move with respect to the backing objective. Therefore, a contact force “penalty” must be paid. In addition, these systems, which are prevalent, exhibit backlash between the sliding elements, which gives rise to “SIL Wobble”—a lack of landing repeatability of the SIL onto the DUT. This prevents the ability to effectively localize small features and/or to assemble multiple images into a mosaic array difficult.
Yet another conventional implementation uses various sensors to determine both the touchdown point and pressure applied towards the object under observation. These sensors are housed in a slide-able mount with respect to the backing objective which in turn is controlled by either a linear or non-linear resistive force. The main focus is on a solidly attached SIL lens and a backing objective housing that adjusts the relationship between the SIL lens and the backing objective. Still another conventional implementation is an optical metrology system focusing on the interferometric and spectroscopic applications using an air bearing type mechanism, in which the lens floats above the wafer via fluid or air. No physical contact is actually made. This method often fails to function properly due to the need for (at least) evanescent coupling of the SIL to the DUT, which cannot be achieved due to surface irregularities and non-flatness and the fact that the design does not allow for an appropriate amount of angular compliance. In addition, the lateral position of the SIL cannot be repeated with a high degree of precision. The high contact force and lack of angular conformance between the SIL and the DUT also degrade the ability of the SIL to be focused at various depths, which is often necessary due to sample preparation issues.
Some conventional implementations of SIL-based objectives draw heat (or drive heat) from (into) the DUT, dropping (raising) the temperature of the DUT in a localized area where the fault, failure, or region of interest is located. These methods are complex and very slow to respond, failing to ensure adequate control of the temperature of the DUT immediately beneath the SIL due to the low thermal impedance between the high conductivity SIL and its housing and the high thermal mass of various components